1. Field of the Invention
The present invention relates to an automatic focus adjusting apparatus which is used in a camera or the like.
2. Related Background Art
Hitherto, in most used in automatic focus adjusting systems of the single-lens reflex cameras, a focal point is set to an object by repetitively executing the cycles of "focal point detection (input of a sensor signal, detection and calculation of a focal point), and lens driving". A lens driving amount in each cycle calculated is based on a defocus amount at the time point when the focal point is detected in the cycle. This is because it is expected that the defocus amount at the time of the focal point detection is eliminated when the lens driving is finished.
Obviously, it takes a certain amount of time to detect the focal point and to drive the lens. However, in the case of a still object, the defocus amount does not change unless the lens is driven. Therefore, the defocus amount to be eliminated upon completion of the lens driving is equal to the defocus amount at the time when the focus detection was executed, and the focal point is accurately adjusted
However, in the case of an object which is moving a high speed, the defocus amount changes during the focal point detection and the lens driving and the defocus amount to be eliminated is remarkably different from the detected defocus amount. Thus, there occurs a problem such that at the end of the lens driving, the focal point is not set to the object.
As automatic focus adjusting methods to solve the above problems, there have been proposed such methods as disclosed in JP-A-62-125311, JP-A-62-139512, JP-A-62-139511, Japanese Patent Application No. 62-293576, and the like.
It is the gist of the methods disclosed in those applications that a change of the defocus amount caused by the movement of an object is foreseen considering a change of the detected defocus amount in each of the foregoing detection cycles and a time interval between the cycles, thereby correcting the lens driving amount. It is expected that the above problems are alleviated by the above methods from the viewpoint of the accuracy of the focal point at the end of the driving of the lens.
FIG. 9 is a diagram for explaining a lens driving amount correcting method according to the above foreseeing method disclosed in Japanese Patent Application No. 62-293576. In the diagram, the of abscissa denotes a time t and the of ordinate indicates an imaging plane position d of an object.
A locus f(t) shown by a solid line denotes an imaging plane position of the object and a locus l(t) shown by a broken line indicates the imaging plane position of the lens.
Explaining in more detail, f(t) denotes the imaging plane position at time t where the object approaches the camera in the direction of the optical axis when the focus adjusting optical system of the photographing lens images the focal point to an infinite position. l(t) represents the imaging plane position of the same object at the focus adjusting optical system position at time t. An interval [t.sub.i, t.sub.i '] corresponds to the focus detecting operation and [t.sub.i ', t.sub.i+1 ] corresponds to the lens driving operation.
Therefore, the difference in the d direction on the ordinate between f(t) and l(t) at the same time t denotes what is called a defocus amount. DF.sub.i indicates a defocus amount detected at time t.sub.i, DL.sub.i denotes a lens driving amount based on the imaging plane position conversion which was executed from the result of the detection of the focal point at time t.sub.i-1, and TM.sub.i represents a time interval of the focus detecting operations.
In the example shown in FIG. 9, as a prerequisite to executing the correcting calculation, it is assumed that the imaging plane position of the object changes in accordance with the quadratic function at.sup.2 +bt+c. That is, at time t.sub.3, if the present and past three imaging plane positions (t.sub.1, f.sub.1), (t.sub.2, f.sub.2), and (t.sub.3, f.sub.3) are known, the imaging plane position f.sub.4 at time t.sub.4 can be predicted.
However, what can be detected by the camera are not the imaging plane positions f.sub.1, f.sub.2, and f.sub.3 but the defocus amounts DF.sub.1, DF.sub.2, and DF.sub.3 and the lens driving amounts DL.sub.1 and DL.sub.2 based on the imaging plane movement amount conversion. Further, the value at time t.sub.4 is a value in the future. Actually, when the accumulating time of an accumulating type sensor changes due to the luminance of the object, such a future value also changes. When an imaging plane position f.sub.4 at time t.sub.4 is determined, it is assumed for simplicity of explanation that the f.sub.4 has already been known from the relation of t.sub.4 -t.sub.3 =t.sub.3 -t.sub.2.
Under such an assumption, the lens driving based on the imaging plane movement amount conversion when executing the lens driving at time t.sub.3 ' toward time t.sub.4 from the result of the focus detection at time t.sub.3 is obtained in the following manner. EQU a.multidot.t.sup.2 +b.multidot.t+c=f(t) (1) EQU a.multidot.t.sub.1.sup.2 +b.multidot.t.sub.1 +c=f(t.sub.1) EQU a.multidot.t.sub.2.sup.2 +b.multidot.t.sub.2 +c=f(t.sub.2) (2') EQU a.multidot.t.sub.3.sup.2 +b.multidot.t.sub.3 +c=f(t) (2")
In FIG. 9, when it is considered that point l.sub.1 is an origin, EQU f.sub.1 =DF.sub.1 ( 3) EQU f.sub.2 =DF.sub.2 +DL.sub.1 ( 3') EQU f.sub.3 =DF.sub.3 +DL.sub.2 +DL.sub.1 ( 3") EQU t.sub.1 =0 (4) EQU t.sub.2 =TM.sub.1 ( 4') EQU t.sub.3 =TM.sub.1 +TM.sub.2 ( 4")
By substituting the equations (3), (3'), (3"), (4), (4'), and (4") for the equations (2), (2'), and (2"), then a, b, and c are obtained. ##EQU1## Therefore, the lens driving amount DL.sub.3 based on the imaging plane movement amount conversion at time t.sub.4 is as follows. ##EQU2##
As mentioned above, the TM.sub.3 is already known from the relation of TM.sub.3 =TM.sub.2 and the DL.sub.3 is obtained from the equation (8). In a manner similar to the above, the lens driving amount at time t after time t.sub.4 can be also obtained from the past three detected defocus amounts DF.sub.n-2, DF.sub.n-1, and DF.sub.n, the past two actual lens driving amounts DL.sub.n-2 and DL.sub.n-1, and the past two time intervals TM.sub.n-2 and TM.sub.n-1. ##EQU3##
If the lens driving amount DL.sub.n to drive the lens is obtained from the detected defocus amount DF.sub.n in accordance with the equations (9), (10), and (11) and the lens is driven on the basis of the DL.sub.n, a focal point can be always accurately obtained at the end of the lens driving even for a moving object.
However, when photographing is executed by actually using the foregoing foreseeing method, the following problems occur.
That is, in the optical system of a camera or the like, when a focal point is set to an object located at a certain distance, it is well known that there is a "focal depth" covering before and behind the imaging plane (focal plane) and within which the image forming state can be regarded to be sufficiently clear for practical use. If the focal depth sufficiently exceeds the effect by the foreseeing operation, the photographing can be executed in a state in which the focal point is accurately set to the object without needing to use the correcting means.
FIG. 2 is an explanatory diagram showing the imaging plane position f(t) of an object V(t) when the automatic focusing operation is executed for an object which approaches toward the camera at a constant speed without using the foregoing foreseeing method and the focal plane position l(t) of the lens obtained by the automatic focusing. The difference e(t)=f(t)-l(t) between the imaging plane position f(t) of the object V(t) and the focal plane position l(t) corresponds to the focusing error. FIG. 3 is an explanatory diagram showing the difference e'(t)=f(t)-l'(t) between the imaging plane position f(t) of the object V(t) and the lens focal plane position l'(t) in the case where the above foreseeing method is used. As will be obvious from FIGS. 3 and 2, there is the relation of e(t)&gt;e'(t) between the focusing errors e'(t) and e(t). The focusing apparatus using the foreseeing method more accurately executes the focusing operation for the object. The difference e(t)-e'(t) becomes the focusing error between the cases where the foreseeing method is used and where it is not used.
As the value of the difference e(t)-e'(t) is larger, the effect of the foreseeing method is larger.
FIG. 4 is an explanatory diagram showing the relation between the difference e(t)-e'(t) and the focal depth for a certain aperture value. E denotes one side of the focal depth.
A deviation at the imaging plane position under the line E corresponds to the region in which practical problems will not occur. Therefore, as shown in FIG. 4, if E sufficiently exceeds the effect e(t)-e'(t) of the foreseeing method, the result equal to or better than that in the case where the foreseeing process wa executed can be obtained without performing the foreseeing process.
As mentioned above, in connection with the focal depth, there is a situation therein the same result of the focusing state is obtained regardless of whether the foreseeing method was used or not. However, in the case of the above apparatus which has conventionally been proposed, when the foreseeing calculating mode is set, the foreseeing calculation is always executed without exception. Thus, there is a drawback such that even in the case where it is possible to obtain a in-focus state without execution of foreseeing calculation as mentioned above, the foreseeing calculation is executed, so that the processing time becomes long.